CELLULOSE CHEMISTRY AND TECHNOLOGY

Cellulose Chem. Technol., 48 (5-6), 455-459 (2014)

HOMOGENEOUS SYNTHESIS OF CROSSLINKED CELLULOSE SPHERES

FROM HEMP (CANNABIS SATIVA L.) STEM AND COTTON

RU YANG, YONG WANG and MIN LI

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology,

Beijing 100029, P.R. China ✉Corresponding author: Ru Yang, yangru@bbn.cn

Received August 8, 2012

Cellulose, extracted from hemp (Cannabis sativa L.) stems, was successfully crosslinked with hexamethylene

diisocyanate through the homogeneous reaction in the solvent N-methylmorpholine-N-oxide (NMMO). Cotton was

also used due to its high cellulose content. The cellulose spheres, about 1 µm in diameter, were obtained through

chemical cross-linking reaction. Fourier transform infrared spectroscopy (FTIR) showed that the carbonyl group,

NH-group and urethane were introduced. Thermogravimetry (TG) indicated that the crosslinked products had better

heat resistance than native cellulose. It was found that the adhesion phenomenon appeared with the increasing amount

of cotton cellulose. A possible model of formation of the crosslinked cellulose spheres was also proposed in this paper.

Keywords: cellulose spheres, hexamethylene diisocyanate, crosslinking, NMMO

INTRODUCTION

Cellulose, the most abundant renewable

resource in the world, is widely used to prepare

cellulose derivatives. Through chemical

modification, cellulose materials with new

interesting properties can be acquired. However,

chemical processing of cellulose is, in general,

difficult because this natural polymer is not

soluble in common solvents, due to its hydrogen

bonds and partially crystalline structure.1

Therefore, traditional modifications are mostly

carried out in the heterogeneous phase. To date,

researchers have focused their attention on

developing the homogeneous way to obtain more

uniform and stable products. Recently, ionic

liquid has been used as a medium in cellulose

chemistry, and homogeneous chemical

modifications of cellulose in ionic liquid have

been reported.2-4

In addition, homogeneous

methylation of wood pulp cellulose dissolved in

LiOH/urea/H2O has also been investigated

lately.5,6

NMMO, as a bulk solvent in industrial

fiber-making processes, can regenerate cellulose

availably. In NMMO medium, cellulose molecu-

lar chains are relatively free and homogeneous

chemical reactions may be carried out. However,

there are still few reports about the chemical

reaction of cellulose in NMMO. Chaudahri et al.7

investigated the carboxymethylation reaction of

cellulose in NMMO system in 1987, and Heinze

et al.8 published an article to give more

experimental details in 1999. Since then, the study

on the homogeneous reaction of cellulose in

NMMO stagnated. As a basic research, it is

necessary to overcome some difficulties to study

the reactions in this cellulose solvent system,

though NMMO has some defects as chemical

reaction medium to some extent. In this paper,

hexamethylene diisocyanate (HDI) was chosen to

crosslink cellulose molecules in the NMMO

system. Isocyanates are a very important

industrial raw material to synthesize elastomers,

such as polyurethane. HDI, whose isocyanate

groups are very sensitive to OH-groups, has been

used to modify cellulose in heterogeneous

system.9

In previous reports, almost all the cellulose

spheres or beads were prepared by a suspension

RU YANG et al.

456

embedding procedure in the presence of Span 80

or other surfactants.10-12

Du et al.13

prepared

cellulose beads from cellulose solution in ionic

liquid by double emulsification formed by Tween

60 and Span 85. The obtained products had a

large size, usually greater than 50 µm in diameter.

There are hardly any reports on cellulose spheres

synthesized by the chemical crosslinking method.

The raw materials used are from hemp stems and

cotton. Therein, cotton consists almost entirely of

cellulose, and hemp (Cannabis sativa L.) has been

planted agriculturally for centuries to get its bast

fiber and hempseed oil. Hemp stems, rich in

cellulose, are usually considered as agricultural

waste. Undoubtedly, this research is one of the

most promising means to convert hemp stems into

novel value-added cellulose materials through

chemical crosslinking.

EXPERIMENTAL Materials

Hemp stems from Yunnan province in China were

smashed into powders (> 60 meshes) and poached

before used. Cotton fibers were obtained from

commercial sources without further treatment. NMMO

(99.7 wt%), purchased from Huai’an Huatai Chemical

Co., Ltd, was dissolved in deionized water to form a 50

wt% solution. Dimethyl sulfoxide (DMSO) was

purchased from Beijing Chemical Works.

Hexamethylene diisocyanate was acquired from

Shanghai Jingchun Reagent Co., Ltd.

Preparation of crosslinked cellulose spheres

Hemp stem powders (0.75 g) were added to 80 mL

of 50 wt% NMMO/H2O solution and soaked with

stirring at room temperature for 4 hours. Then, the

mixture was heated to 90 oC to remove water through

reduced pressure distillation, making the cellulose

dissolve out of the hemp stems easily. The temperature

was maintained at 90~95 oC for 30 min and a sticky

light-yellow translucent solution was obtained. The

insoluble substance was filtered with a fine screen,

washed, dried and weighed. Subsequently, the filtrate

containing 0.15 g cellulose extracted from hemp stem

was diluted with 35 mL of DMSO. Then, 2 mL of HDI

was added dropwise into the diluted cellulose solution

(NMMO/DMSO) under strong agitation for one hour.

To separate out the final products, 100 mL of

deionized water was added to this cloudy mixture. The

light yellow powder was named HC-HDI. If water was

directly added without HDI, hemp stem cellulose (HC)

could also be acquired.

The preparation process of cotton cellulose (CC)

solution was the same as described above. Likewise,

0.15 g and 0.5 g cotton fibers were, respectively,

dissolved in NMMO solution without filtering the

insoluble substance and 35 mL of DMSO was used to

dilute the solution. After that, 2 mL of HDI was added

dropwise into both diluted cellulose solutions under

strong agitation for one hour. One hundred milliliters

of deionized water was added to the cloudy mixture,

and the final products were named CC-HDI-0.15 and

CC-HDI-0.5, respectively.

The detailed preparation information is provided in

Table 1.

Measurement

The morphology was observed by a HITACHI

S4700 scanning electron microscope (SEM). Fourier

transform infrared spectra (FTIR) were acquired to

investigate the chemical structure and groups with a

Nicolet 8700 FTIR spectrometer. Thermogravimetric

(TG) experiments were carried out under a nitrogen

flow with a TG/DTA thermal analyzer from Shanghai

Precision & Scientific Instrument Co., Ltd.

Table 1

Detailed preparation information of the three samples

Samples Cellulose weight (g) NMMO (mL) DMSO (mL) HDI (mL)

HC-HDI 0.15 80 35 2

CC-HDI-0.15 0.15 80 35 2

CC-HDI-0.5 0.50 80 35 2

RESULTS AND DISCUSSION

SEM study of the morphology

Fig. 1 exhibits the SEM images of cellulose

raw materials and their corresponding spheres.

Fig. 1a shows the cellulose extracted from hemp

stems, which appears to be interlaced and

ribbon-like. However, the HC-HDI samples

synthesized from the crosslinking reaction with

HDI are spherical and about 1 µm in diameter

(Fig. 1b). These spheres look like clews woven of

microfibers in the magnified image (Fig. 1c). Fig.

1d, e and f show the morphology of crude cotton

fibers, CC-HDI-0.15 and CC-HDI-0.5,

respectively. It can be seen that the cotton

Cellulose

457

cellulose in Fig. 1d is fibriform, of about 10 µm in

diameter, while the same clewlike spheres as

HC-HDI are also exhibited in Fig. 1e and f.

Moreover, with the increasing amount of cotton

cellulose, the adhesion phenomenon appeared

possibly owing to the excess cellulose in solution.

Figure 1: SEM images of (a) cellulose extracted from hemp stems, (b) HC-HDI, (c) magnified HC-HDI, (d)

cotton cellulose, (e) CC-HDI-0.15 and (f) CC-HDI-0.5

Figure 2: FTIR spectra of hemp stem cellulose and its crosslinked products with HDI (a), and cotton cellulose

and its crosslinked products with HDI (b)

Chemical groups characterized by FTIR

Fig. 2 shows the FTIR spectra of pristine

cellulose and the reaction products with HDI.

Both the regenerated cellulose from hemp stems

in Fig. 2a and the cotton cellulose in Fig. 2b

exhibit typical absorption peaks of cellulose,

which were reported before.14

After the

introduction of HDI, both HC–HDI and CC–HDI

show similar FTIR spectra. An intensive band for

the –OH (and –NH) groups in the range of 3400

cm-1

is also observed. However, this peak became

narrower and the relative absorbance decreased

with the reduction of cellulose from 0.5 g to 0.15

g (Fig. 2b). That may be attributed to the

sufficient reaction of isocyanate groups with

hydroxyl groups on the cellulose chains when

there was less cellulose.1 The double peaks at

2933 cm-1

and 2857 cm-1

show symmetric and

asymmetric –CH2– stretches, which were

collectively induced by cellulose and HDI.15

Further, the peaks at 1622 cm-1

and 1574 cm-1

can

be related to the appearance of carbonyl group

(C=O) and the NH-group deformation oscillation,

which confirms that cellulose was crosslinked

with HDI.9,15,16

The peak at 1254 cm-1

is assigned

to the C–O group connecting with Pi bond, which

further indicates the presence of urethane

introduced by the reaction of –OH with

–N=C=O.15,16

RU YANG et al.

458

Possible formation mechanisms of the cellulose

spheres

According to the functional group analysis, a

crosslinking reaction schematic is proposed in Fig.

3. Cellulose was dissolved in the NMMO system,

which was diluted by DMSO. HDI was slowly

dropped into the solution under stirring. The HDI

droplets were dispersed in this solution just like

an emulsion system. The cellulose molecules

around droplets reacted with HDI rapidly by

following procedures (A) and (B), resulting in the

crosslinked clewlike spheres. In procedure (A), a

urethane linkage was formed through the reaction

between isocyanate and a hydroxyl group. The

urethane group continued to react with a new

isocyanate molecule to generate an allophanate

group in procedure (B).

Figure 3: A proposed model for the formation of crosslinked cellulose spheres

Figure 4: TG curves of hemp stem cellulose and its crosslinked products with HDI (a), and cotton cellulose and

its crosslinked products with HDI (b)

Thermostability analysis

Thermogravimetric curves for pristine

cellulose and their crosslinked products with HDI

are shown in Fig. 4. The curves of both hemp

stem cellulose and HC-HDI had an initial loss of

moisture and desorption of gases at about 80-110 oC, as shown in Fig. 4a. Next, a major

decomposition proceeded from 230 to 370 oC for

hemp stem cellulose and its maximum

decomposition temperature appeared at 340 oC.

However, for HC-HDI, there were two additional

weight losses at 180-280 oC and 380-450

oC,

which could be attributed to the actual pyrolysis

brought by the minor decomposition reaction1 and

the breakage of the crosslinked structure,

respectively. The final decomposition temperature

of HC-HDI is higher than that of the original

cellulose, because the crosslinked structure can

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459

improve the thermal stability.17

In Fig. 4b, the TG

curve of cotton cellulose shows one more weight

loss at 370-520 oC, compared to the regenerated

hemp stem cellulose, which may attributed to the

better crystalline region and less amorphous

region in the untreated cotton cellulose.18

Yet, the

final thermal stability temperatures of the reaction

products are lower than those of the untreated

cellulose, possibly because the crosslinked

structure of regenerated cotton cellulose from

NMMO is still less thermoduric than the perfect

natural cotton.

CONCLUSION

Crosslinked cellulose spheres, about 1 µm in

diameter, have been prepared by the

homogeneous reaction of plant cellulose with

HDI in the NMMO system. The characterization

results imply that these products have crosslinked

structure of urethane and allophanate groups. It

was found that the products from hemp stems are

more thermoduric than the regenerated cellulose,

but the result is opposite for cotton cellulose.

Finally, it should be pointed out that NMMO

must be handled with caution, as it is oxidizing,

labile and even explosive under high temperature,

and any incorrect operation may lead to hazards.

REFERENCES 1 C. Yin, J. Li, Q. Xu, Q. Peng, Y. Liu and X. Shen,

Carbohyd. Polym., 67, 147 (2007). 2 Z. M. Wang, L. Li, K. J. Xiao and J. Y. Wu, Bioresour.

Technol., 100, 1687 (2009).

3 K. Schlufter, H. P. Schmauder, S. Dorn and T. Heinze,

Macromol. Rapid Commun., 27, 1670 (2006). 4 T. Erdmenger, C. Haensch, R. Hoogenboom and U. S.

Schubert, Macromol. Biosci., 7, 440, (2007). 5 J. Cai and L. Zhang, Macromol. Biosci., 5, 539

(2005). 6 M. C. V. Nagel, A. Koschella, K. Voiges, P.

Mischnick and T. Heinze, Eur. Polym. J., 46, 1726

(2010). 7 S. N. K. Chaudhari, K. C. Gouden, G. Srinivasan and

V. S. Ekkundi, J. Polym. Sci. Polym. Chem. Ed., 25,

337 (1987). 8 T. Heinze, T. Liebert, P. Klufers and F. Meister,

Cellulose, 6, 153 (1999). 9 T. G. Gafurov, M. Y. Pilosov, A. Adylov et al., Polym.

Sci. USSR, 12, 2848 (1970). 10

H. Fang, J. Wei and Y. Yu, Biomaterials, 25, 5433

(2004). 11

Y. Tang, Q. Zhang, L. Wang, P. W. Pan and G. Bai,

Langmuir, 26, 11266 (2010). 12

X. Luo and L. Zhang, Biomacromolecules, 11, 2896

(2010). 13

K. F. Du, M. Yan, Q. Y. Wang and H. Song, J.

Chromatogr. A, 1217, 1298 (2010). 14

Q. Gao, X. Shen and X. Lu, Carbohyd. Polym., 83,

1253 (2011). 15

P. Stenstad, M. Andresen, B. S. Tanem and P. Stenius,

Cellulose, 15, 35 (2008). 16

K. Wilpiszewska and T. Spychaj, Carbohyd. Polym.,

70, 334 (2007). 17

C. Chen, L. Wang and Y. Huang, Mater. Lett., 63,

569 (2009). 18

Abd-Alla M. A. Nada, S. Kamel and M. El-Sakhawy,

Polym. Degrad. Stabil., 70, 347 (2000).